Measuring and testing – Gas analysis – Gas of combustion
Patent
1994-07-27
1996-02-27
Williams, Hezron E.
Measuring and testing
Gas analysis
Gas of combustion
73 3106, G01N 2712
Patent
active
054938961
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is based on a sensor arrangement for determining gas components and/or gas concentrations of gas mixtures of the generic type of the main claim.
2. Description of the Related Art
Sensors for determining the content of carbon monoxide in gas mixtures on the basis of semiconducting metal oxides which at high temperatures, as a function of the carbon monoxide content, but also of the oxygen content, change in electroconductivity are known (German Patent 2603785). One metal oxide used is, for example, tin oxide which at elevated temperatures adsorbs oxygen at its surface and ionizes it negatively. As a result, a potential threshold is formed on the grain boundaries of the tin oxide, which threshold grows with increasing concentration of the oxygen adsorbed in ionic form. If the tin oxide surface covered with oxygen then comes into contact with oxidizable gases such as, for example, CO, these are oxidized by the adsorbed oxygen. In the process, the negative charge at the interface and thus the potential threshold is lowered. The surface conductivity of the metal oxide therefore rises again. If the conductivity change is to be utilized for measuring the concentration of oxidizable gas components such as, for example, CO, NO.sub.x and HC, there thus results an oxygen cross sensitivity. In addition, at low oxygen partial pressures and, in particular in association with high temperatures, irreversible reduction of the semi-conducting metal oxide and thus failure of the sensor occurs. Hereinafter, the gas components to be determined are denoted as pollutant components.
SUMMARY OF THE INVENTION
The invention is based on the insight that, if the grain faces of the semiconducting metal oxide are overcrowded with oxygen, even high oxygen concentration changes have a negligibly small effect on the sensitive zone of the measuring element.
The sensor arrangement according to the invention, having the characteristic features of the main claim, has the advantage that by simple means the cross sensitivity of the measuring element with respect to oxygen is largely eliminated. Owing to the simplicity, the sensor arrangement can be fabricated cost-effectively. Moreover, no oxygen pump cell is necessary which pumps oxygen to the measuring element.
As a result of the measures listed in the subordinate claims, advantageous refinements and improvements of the sensor arrangement specified in the main claim are possible. It is particularly advantageous to set, by introducing diffusion barriers, a concentration gradient, sufficiently opposite in direction, of pollutant component and oxygen. If, owing to the exhaust gases of other motor vehicles, a high CO concentration is present in the external air used as the reference gas, an oxidation catalyst is used to oxidize the CO with the air oxygen present in excess. As a result, the CO concentration is reduced to negligible values on the reference gas side, whereas the O.sub.2 concentration remains constant, as a first approximation.
BRIEF DESCRIPTION OF THE DRAWINGS
A specific embodiment is described in the invention and shown in more detail in the following description.
FIG. 1 shows a sectional view of the sensor arrangement according to the invention and FIG. 2 shows the variation of the oxygen and the CO concentration over the length of the diffusion duct according to FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
According to FIG. 1, the sensor arrangement consists of a ceramic support 20 made of, for example, Al.sub.2 O.sub.3, with a bottom ceramic support part 21 and a top ceramic support part 22. Between the ceramic support parts 21, 22 there is a diffusion duct 23 with an exhaust gas-side orifice 24 and a reference gas-side orifice 25. Toward the exhaust gas-side orifice 24 there is formed an exhaust gas-side diffusion section 26, and toward the reference gas-side orifice 25 a reference gas-side diffusion section 27. Inserted in the exhaust gas-side diffusion section 26 there is a porous exhaust gas-sid
REFERENCES:
patent: 5070721 (1991-12-01), Tantram
Database WPI, Week 9116, Derwent Publications Ltd., London, GB; AN 91-115079 & JP-B-3022584 (Toshiba KK), Mar. 27, 1991.
Brook Michael J.
Robert & Bosch GmbH
Williams Hezron E.
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